1. Field of the Invention
This invention relates generally to a system and method for determining whether an anode pressure sensor in a fuel cell system is operating properly and, more particularly to a system and method for determining whether an anode pressure sensor in a fuel cell system is operating properly by comparing the anode pressure sensor reading to the sum of an ambient pressure sensor measurement and a differential pressure sensor measurement between the anode and the cathode.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of and back into the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As is well understood in the art, the membranes within a fuel cell need to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. This humidification may come from the stack water by-product or external humidification. During operation of the fuel cell stack, water by-product and external humidification may enter the anode and cathode flow channels, and may accumulate within the anode or cathode gas flow channels.
Typically, it is necessary to operate a fuel cell stack so the anode side of the stack is at a slightly higher pressure than the cathode side of the stack. Reasons for keeping the anode side pressure slightly higher than the cathode side include reducing the amount of nitrogen crossover that occurs across the MEA and preventing cathode exhaust gas from getting in the anode side during a bleed event. In order to insure that the proper pressures are present, the fuel cell system typically employs a cathode pressure sensor for measuring the pressure of the cathode side of the fuel cell stack, an anode pressure sensor for measuring the pressure of the anode side of the fuel cell stack, and a differential pressure sensor for measuring the pressure difference between the cathode side and the anode side of the fuel cell stack.
Liquid water in the anode and cathode side of the fuel cell stack may freeze and form ice. Sometimes this ice can form on and around the anode side pressure sensor, which affects its ability to provide a proper pressure reading. Therefore, during the next system start-up, a frozen anode pressure sensor may give an inaccurate reading indicating that the anode side pressure is too high or too low, or a proper pressure measurement when the anode side pressure is actually too high or too low. If an improper anode side pressure reading is given and the anode pressure is too high, excess hydrogen could be lost and system failures could occur as a result of components breaking. A low pressure anode side could lead to a rapid destruction of the cathode side catalyst.